The present invention relates to a planar monolithic semiconductor structure of a bipolar heterojunction transistor and a laser. This transistor forms part of the control or modulation circuit of the laser. The invention mainly applies to the field of telemetry, integrated optics or optical fibre telecommunications.
More specifically, the invention relates to a planar, bipolar, integrated heterojunction transistor--laser structure produced on a semiconductor substrate of III-V material.
As a known integration solution for a heterojunction bipolar transistor and a laser source, reference can be made to the article in Appl. Phys. Lett, 37(2) of July 15, 1980, pp 211-213 and entitled "A monolithic integration of GaAs/GaAlAs bipolar transistor and heterostructure laser" by J. KATS et al. In this integration solution, the laser and bipolar transistor are juxtaposed on the same GaAs semiconductor substrate on which are epitaxied the different semiconductor layer in which are defined the junctions of the laser and the transistor. In this monolithic structure, the laser has a relatively high threshold current and therefore a high energy dissipation, together with a low differential efficiency, so that a high modulation current is required.
As another solution for the integration of a laser and a transistor, reference can be made to the article in Appl. Phys. Lett, 46(3) February 1985, pp 226-228 entitled "Monolithic integration of Low Threshold current quantum well laser and a driver circuit on a GaAs substrate" by T. SANDA et al. In this solution, the transistor used is a field effect transistor. In addition, the laser is of the type having a vertical injection of the electrons and the active layer of the laser is provided with a GRINSCH.
The advantages of such a laser structure are in particular low threshold current and consequently a low energy dissipation, so that only a low modulation current is required. moreover, the corresponding field effect transistor--laser structure has a limited thermal sensitivity and can operate at high temperatures.
Thus, this integrated field effect transistor--laser structure has performance characteristics very close to those of discrete components. However, for producing a short gate transistor and therefore a high transconductance transistor, it is necessary to form the laser at the bottom of a hole. This technology is relatively complicated and in particular causes problems with respect to the production of resin masks by photolithography and which are used for defining the dimensions of the transistor gate. It is difficult to reproduce this technology. Furthermore, for increasing the reliability of transistor production, the semiconductor layers of this component must be produced following the epitaxy of the laser conductor layers during a supplementary operation. The laser performance characteristics may be impaired by said repeated ionic epitaxy operations.